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Old 03-08-2002, 09:19 PM
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Join Date: Feb 2001
Location: BC, Canada
Posts: 378
5 speed 300E Idles at 1500rpm

My car has begun idling at 1500 rpm even when fully warmed up. The engine temperature is normal and the car is running fine. Sometimes, the idle speed will drop to 900 or so by itself, but it will often remain at 1500. This isn't a huge problem in drivability except that it causes the engine speed to remain high during shifting. This is a major problem when I am in a big hurry. I even blew a shift into second once when I was showing off a bit.
So, I'm going to get working on the throttle linkage, but I wondered if there was an obvious place to start first.
1967 250SL
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Old 03-09-2002, 12:42 AM
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Join Date: Jun 1999
Location: Surrey, Beautiful British Columbia, Canada
Posts: 750
check your OVR! do a search, there are numerous
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Old 03-09-2002, 10:08 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
On the 300E the idle would be at 500RPM or less if the overvoltage relay was bad.
There are four sensors involved with correct idle speed on this car. Sounds like at least one isn't sending correct info the CIS-E computer.
These sensors include idle contact, decel swittsch, air flow position sensor, & hall-effect speed sensor.
A test of these imputs to the ecu will lead to correct diagnosis & repair.
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Old 04-04-2002, 06:59 PM
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Join Date: Feb 2001
Location: BC, Canada
Posts: 378
Well, I got the bad news. The list of parts to replace include:

- oxygen sensor
- air flow sensor
- throttle position switch (sensor?)
- throttle body
- throttle cutoff switch

The air flow sensor and the throttle body are very expensive.
How do they work? I think that it's time for me to get the
Bosch fuel injection book. If I get these parts fixed, my paint
job will have to wait a long time.
1967 250SL
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Old 04-05-2002, 07:43 AM
Posts: n/a
I have the same problem with my '86 5-speed or at least I thought I did. I would take 20 minutes of driving before the idle would settle down to normal and that was not until the temp guage read around 90C. May be it has a low temp thermostat or the thermostat is sticking open and not letting the car warm up as fast as it should. A low engine temp would signal the computer to fast idle the engine. Up in B.C. your car may not get hot enough.
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Old 05-01-2002, 06:33 AM
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Join Date: Nov 2000
Location: Umeň,Sweden
Posts: 46
Have you checked for air leaks ?
If you have a vacuum leak the idle rpm can get high.
check all rubber lines
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Old 05-01-2002, 11:15 AM
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Join Date: Feb 2001
Location: BC, Canada
Posts: 378
The throttle cutoff switch was replaced which was about a $25 part. The idle is now excellent at 900rpm. The oxygen sensor is not showing any voltage and needs to be replaced. The air flow sensor is likely fine but the throttle body switch is not responding correctly at all positions.
I'm happy with it now, though.
1967 250SL
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